This research aims to develop improved composite dental restorative materials. In phase I, high molecular weight siloxane monomers were used to prepare visible light-activated composites, and the composites were evaluated according to ADA specification No.27. Hardness, diametral tensile strength, and water sorption of the composites were determined as well as the degree of conversion of the siloxane monomers which were determined by ATR infrared spectroscopy. Among the siloxane monomers studies, a siloxane monomer showed a high degree of conversion, and a low water sorption. Further, the high molecular weight monomer does not require a diluent, and mechanical properties of the siloxane composites are comparable to those based on Bis-GMA. In phase II, the composite formulations will be further optimized by determining the optimum filler composition and concentration; by studying the polymerization shrinkage, microleakage, and contraction stress relaxation of the composites; by determining the wear of the composites in artificial oral environment; and by improving the color stability of the composites. The major aim is to develop improved composite materials which are suitable for posterior restorations.